Phase transition type change in order-order and order-disorder transformations of LaH2+c-like superstoichiometric dihydrides

Phase transformations in metal-hydrogen systems associated with the ordering processes in the subsystem of interstitial hydrogen atoms are considered. It is shown that in a particular class of superstoichiometric rare-earth dihydrides, where the ordering subsystem is characterized by two long-range-order parameters η1, η2 and the energy parameter p < 1, the order-order transition can change its type: being continuous for concentrations c within the range c0(1)(p) ≤ c ≤ c0(2)(p), it becomes discontinuous outside of this region. Dependence of critical concentrations c0(1) and c0(2) upon the energy parameter p is deduced. It is shown, as well, that there exists a second sequence of critical concentrations, c1(1)(p) and c1(2)(p), denoting the boundaries between the regions of continuous and discontinuous order-disorder transformations. (orig.)